Check that there are no uncommitted changes in the release script.

Any such changes wouldn't be included in the release, which is probably
unexpected.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77965 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

build/tools/git-make-release

@@ -38,6 +38,11 @@ if [ ! -r "$EOL_FILE" ]; then
     exit 1
 fi
 
+if ! git diff --quiet; then
+    echo "Working copy has modifications, commit or stash them." >&2
+    exit 2
+fi
+
 set -e
 set -x